A new study concludes that hundreds of young children in the District experienced potentially damaging amounts of lead in their blood when lead levels were dramatically rising in the city's tap water.

In some high-risk neighborhoods, the number of toddlers and infants with blood-lead concentrations that can cause irreversible IQ loss and developmental delays more than doubled after harmful levels of lead began leaching into the city's drinking water in 2001, according to the findings. The peer-reviewed study, obtained by The Washington Post, is to be published soon in Environmental Science and Technology, a journal on advances in chemical and environmental research.

Authors of the study, at Virginia Tech and Children's National Medical Center, said their findings raise concern about the 42,000 D.C. children, now ages 4 to 9, who were in the womb or younger than 2 during the water crisis. Those children might be at risk of future health and behavioral problems linked to lead, the report said.

The study, based on a detailed analysis of thousands of children's blood tests from 2000 to 2003, contradicts the public assurances issued by federal and D.C. health officials starting in 2004. At the time, although officials acknowledged that the amount of lead in city water were at record-breaking levels, they said repeatedly that they found no measurable impact on the general public's health.

The lead concentrations in the city's water were sometimes hundreds of times higher in individual homes than the amount the federal government considers a level of concern. The lead concentrations began rising in 2001, after a new chemical was added to the water treatment, and they persisted until they were publicized in a February 2004 Post article.

The current researchers found the most dramatic rise in the number of children with unsafe blood-lead levels in Mount Pleasant and Columbia Heights, the southeastern portion of Capitol Hill, a large swath of Ward 4 along Georgia Avenue, and Northeast Washington's Langdon Park. According to a Post analysis of lead tests in 2003 and 2004, these were all neighborhoods where some of the highest levels of lead were measured in tap water during the crisis. Those areas also relied heavily on lead pipes.

Specialists say damage from lead is often permanent. Children can exhibit signs of aggressiveness and difficulty focusing in school. Numerous studies show lead-poisoned children can on average lose 3 to 7 IQ points. The harm can be mitigated, however. Doctors say parents should focus on providing children with a healthful, calcium-rich diet and an enriching educational environment that includes reading to them regularly. There is no blood test to detect years-old lead exposure, but parents are encouraged to monitor their children.

The D.C. Water and Sewer Authority said the most recent tests of the city's water show lead levels in the safe range, with an average reading of 8 parts per billion or lower, in a group of homes. That is far below the federal level of concern of 15 parts per billion and is the result of a chemical treatment added in 2004.

Public health officials said this week that their earlier statements and reports did not try to suggest that the city's lead crisis had no impact on health but that at the time they had no data to indicate a problem. Some said they would have liked to do more research on possible links but ran into obstacles. Federal agencies and WASA cited a 2004 Centers for Disease Control and Prevention report as having "confirmed that there was no identifiable public health impact from elevated lead levels in drinking water."

The CDC relied on a broad, wide-ranging look at citywide tests for children and adults. The new study looks more specifically at children most vulnerable to lead poisoning and focused on neighborhoods with the highest levels of lead in water.

Mary Jean Brown, the CDC's lead poisoning prevention director, said she welcomes the new research and said the CDC's 2004 report noted concerns about a possible health impact but found no direct link. She said this week that she cannot control how other public agencies and officials might have interpreted the 2004 report.

"If people are walking away from this and saying there's no problem with drinking water, I don't know how to respond to that," she said. "We were under some constraints . . . working as quickly as we could. We didn't find evidence of a public health crisis. We used the methods at our disposal at the time."

The 2004 report found that of 201 residents tested in homes with unusually high lead in the water, "all had blood lead levels below CDC's concern." It also found that average blood levels did not rise measurably in the city but that D.C. residents in homes with lead pipes did not follow the national trend of lower blood-lead levels.

WASA General Manager Jerry Johnson said he could not comment on the new study because he has not seen it but said he relied in the past on the advice of trained scientists as WASA tried to disseminate all the information it could as fast as possible.

"I was quoting information that was provided to us by health experts with the very best information we had at the time," he said.

A spokesman for Mayor Adrian M. Fenty (D) said the mayor would look at the study when it is released and consider how to use it to continue improving the District's response to public health problems.

D.C. Attorney General Peter J. Nickles said yesterday that "obviously we'll take a close look at the study, and if the science demonstrates we need to do more for the public, we'll do more. Mayor Fenty takes the issue of lead in water very seriously."

The researchers in the latest report, based on a subset of tests reviewed by the CDC, identified hundreds of specific children whose blood lead rose to harmful levels during the city's lead crisis and generally fell when the water lead levels fell.

The correlation of timing and geography, several experts not involved in the study said, increases the likelihood that leaded water contributed to the changes in blood readings.

"There is no doubt that many children in this city were profoundly impacted by the years of completely unnecessary exposure to high lead in the District's water," said co-author Marc Edwards, a Virginia Tech engineer and MacArthur scholar. "We hope this study will stop future harm and address the misrepresentations and false statements about what really happened." Edwards has been a dogged critic of the government's role in creating the lead crisis and its later response.

Children younger than 2 and fetuses are the most vulnerable to permanent damage from lead, experts agree, because their brains are still developing and because they tend to both ingest and absorb much more of the toxic metal than do adults or older children, based on their body weight. Federal health officials have long viewed lead paint as the most dominant source of lead poisoning for children. But because the District's tap water had the highest lead readings ever measured in the nation, many experts have become increasingly concerned.

"I was surprised by how high the blood-lead levels were," said study co-author Dana Best, a Children's National Medical Center pediatrician and epidemiological researcher. "And by the time there are measurable levels, the damage has been done. We cannot continue to use children as the canaries in the coal mine. We have to stop testing children to determine what danger is in the environment and start testing the environment to make sure children don't get harmed."

Some parents, local activists and lead experts said the new research raises questions about the sweeping statements of public officials -- and the thoroughness of preliminary reports.

"Residents with high lead levels in their tap water did not have elevated blood lead levels, " the U.S. Environmental Protection Agency said in one fact sheet, citing the 2004 CDC report.

The news of the study came the day after the new EPA administrator, Lisa P. Jackson, pledged in a letter to employees that protecting children from toxins would be one of her top priorities in office.

David Bellinger, a well-known neurologist at Harvard University who focuses on the effects of lead and other toxin exposure on children, said he is struck that the number of young children with unsafe blood lead levels more than doubled in some targeted neighborhoods during the lead crisis.

"That's a big increase, and even the lower increase for kids with moderate risk is reason for worry," he said. "Compared to other kids in the U.S., these kids [in D.C.] were getting exposed to a lot more lead than other preschoolers."

If earlier reports were so limited in their scope, researchers should have avoided sweeping conclusions, Bellinger said.

"If these data were available previously, I would be surprised that anyone would be assuring the public there was no problem," he said. "It's hard to say in hindsight how much they should have known, or how deeply they should have probed. But without looking at the study, it surely sounds like the earlier statements made were falsely reassuring."

The CDC study was limited, according to public records and some environmental researchers.

CDC researchers tested the blood lead of 201 people in 2004. The residents had been warned in 2003 of unusually high lead levels in their tap water. They had been advised to use alternative sources for drinking water, and all the children were using bottled and filtered water when tested, according to their parents, and were thus unlikely to have high blood-lead levels.

Some of the public health officials said this week they had wanted later to analyze the possible links between blood lead and leaded water. Tee Guidotti, a George Washington University environmental health professor who was under contract as WASA's health adviser, said he stands by his findings in a 2007 report that average blood-lead levels were generally falling in 2003 and 2004, when water lead levels were generally rising.

"Do I wish we were better understood? Do I wish certain things were said differently? Do I wish we'd had better data? Sure," Guidotti said. "But those things were out of my control. In terms of what was under my control, I have no regrets about my role."

Walter Smith of the nonprofit advocacy group D.C. Appleseed Center for Law and Justice said his organization is still pushing for independent testing of the water.

Of the report, he said, "This casts further doubt on the reliability of the agencies who told us not to worry and are still telling us not to worry. We're talking about the health of little children. It's not like this is some insignificant issue."